A steel wire for a spring and a production method therefor are disclosed in, for example, Japanese Examined Patent Publication No. 2-35022. According to the technique proposed in this case, a surface layer part of the steel wire is repeatedly subjected to a cycle of rapid heating and rapid cooling so as to be self-cooled by using the temperature difference between the surface layer part and a center part of the steel wire. As a result, crystal grains of the surface layer part are fined without being cooled forcedly. In addition, the heating cycle is repeated until the temperature at the center part exceeds the A1 transformation point, whereby an entire cross section of the steel wire is made to have a martensite structure.
Another technique is disclosed in Japanese Examined Patent Publication No. 7-91585. In this technique, a steel wire is quenched by heating and rapid cooling in a particular pattern in a heating condition (temperature and cooling rate) so that only the surface side thereof is transformed into quenched martensite. Then, the steel wire is reheated and is warm coiled while being tempered, whereby compressive residual stress is generated in the surface due to transformation strain of the martensite of the surface layer.
Regarding reduction in the dimensions and in the weight of suspension springs, high design stress is required, and spring materials should be greatly strengthened in view of sag resistance and durability in responding to the requirement for high design stress. However, when the strength is increased, the delayed fracture sensitivity and the sensitivity to defects such as corrosion pits generated by snow melting material would be increased. Therefore, alloys have been developed by adding large amounts of elements such as Ni, Cu, Cr, Ti, V, etc. thereto so as to decrease the above environmental embrittlement sensitivity. These alloys have low versatility, and the material costs thereof are high, compared with SUP7, SUP12, and the like.
On the other hand, it is publicly known that fining crystal grains is effective as a method for improving the environmental embrittlement resistance. In order to fine crystal grains, a method of rapid heating and rapid cooling is effective, and techniques of using high frequency quenching may be used. Moreover, in order to use springs under high design stress in view of decreasing the weight of the springs, the hardness of the springs should be increased so as to obtain high sag resistance. However, when the hardness is increased, the rate of crack propagation is increased, and the fatigue characteristics are degraded.